The image forming apparatuses, such as copy machines, printers and facsimile machines, applying the electrophotographic techniques may encounter image density variations of a toner image due to individually different characters of apparatuses, variations with time, or changes of conditions surrounding the apparatus which include temperature, moisture and the like. Heretofore, there have been proposed a variety of techniques for ensuring a stable image density, which include, for example, a technique wherein a small test image (patch image) is formed on an image carrier such that a density control factor affecting the image density may be optimized based on the density of the patch image. This technique takes the following approach to attain a desired image density. That is, predetermined toner images are formed on the image carrier with the density control factor set to a different value each time while the image density of each toner image, as the patch image, borne on the image carrier or transferred onto another transfer medium such as an intermediate transfer medium is detected. Subsequently, the density control factor is adjusted so as to establish coincidence between the density of the patch image and a previously defined target density.
Heretofore, there have been proposed a variety of techniques for taking measurement of the patch image density (hereinafter, referred to as “patch sensing technique”). Above all, the technique based on optical means is most commonly used. Specifically, light is irradiated on a surface area of the image carrier or the transfer medium with the patch image formed thereon, while reflection light or transmission light from the surface area is received by an optical sensor. Then, the density of the patch image is determined based on the amount of received light.
In the image forming apparatus adapted to adjust the density control factor based on the density of the patch image, how accurately the density of the formed patch image is detected is an important point for obtaining the toner image of good image quality by way of the density control factor set to an adequate value. However, the above conventional patch sensing technique does not take direct measurement on the density of the formed image but merely provides an estimation of the image density consequentially derived from a detected result of the amount of light from the toner image as the patch image temporarily borne on the surface of the image carrier or the transfer medium. Therefore, it may not necessarily be said that the sensor output correctly represents the final image density. In addition, there may be a case where variations in the characteristics of the sensor or detection errors result in inconsistency between the sensor output and the final image density.
Where the image density of the toner image formed on the image carrier such as a photosensitive member or the transfer medium is measured by means of a density sensor as described above, the measurement result does not simply depend upon the amount of toner adhered to the image carrier but may be varied depending upon surface conditions of the image carrier which include reflectivity, surface roughness and the like. If the surface color of the image carrier is altered as the cumulative sum of prints produced by the image forming apparatus increases, for example, the output from the density sensor is varied in accordance with the change in the surface color of the image carrier even though the same amount of toner is adhered thereto. This results in disability to take accurate density measurements. In addition, where the image carrier has inconsistent surface conditions, influences of such surface conditions cannot be ignored.
If the sensor output does not accurately represent the final image density as described above, the density control factor is adjusted based on the image density erroneously estimated from such a sensor output. As a result, the density control factor is set to a value deviated from its optimum value. Particularly in a state where the toner is adhered to the image carrier in a relatively high density like when a solid image is formed thereon, for example, the final image density is varied less relative to the degree of increase or decrease of the amount of toner adhesion. Accordingly, even a minor deviation of the sensor output entails a significant deviation of the value of the density control factor defined based on such a sensor output. Consequently, the density control factor is set to a value significantly deviated from its optimum value so that the image quality is degraded and the following problems may also occur in cases.
In a case where, for example, the image density of a high-density image like a solid image is estimated from the sensor output to be a value lower than an actual image density thereof, the apparatus adjusts the density control factor in a manner to further increase the image density. As a result, an excessive amount of toner is made to adhere to the image carrier so as to cause a transfer/fixing failure or to increase toner consumption abnormally. In addition, while the image forming process is repeated under the condition that the amount of toner adhesion is increased more than necessary, the preceding image forming processes will leave cumulative adverse effects on an image to be formed subsequently or the service life of the apparatus may be shortened notably.
In addition, the image density of the patch image to be formed depends upon a combination of various factors and hence, complicated processings are required for discretely optimizing the plural density control factors affecting the image density based on the image density of the patch image. The conventional density control techniques have problems associated with the increased cost of the apparatus burdened with such complicated processings and the decreased throughput of the image formation suffering the time-consuming processings. In this connection, demand exists for the establishment of a technique for reliably optimizing the density control factor in a more simplified manner.
It is a first object of the present invention to provide an image forming apparatus and method adapted to set the density control factor in a proper state as excluding the influence of detection errors of the patch image density which result from the variations of the characteristics of the sensor or the like.
It is a second object of the present invention to provide an image forming apparatus and method adapted to optimally set the density control factor based on the image density of the toner image thereby ensuring stable formation of the toner image of good image quality.
It is a third object of the present invention to provide a density control technique suitable for an image forming apparatus of a non-contact development system.